Recent investigations have demonstrated that UV far (ultraviolet) radiation produced by an excimer laser at a wavelength of 193 nm can ablate (remove) corneal tissue with minimal trauma, loss of transparency or scarring. The ablative process that occurs using this far UV wavelength region primarily results from the breakup of intramolecular bonds as reported by Trokel, et. al., in the American Journal of Ophthalmology, Volume 96, No. 6, December, 1983, Entitled: "EXCIMER LASER SURGERY OF THE CORNEA". This action is in contrast with that produced by higher wavelength laser radiation in the infrared region where tissue destruction results from thermal heating and the precision of tissue removal is much degraded.
The lens of the eye will absorb all 193 nm UV radiation by the application of Beer's law of light transmission given by T=10.sup.-ad, where T is the transmission (100%=1.0), a is the absorbance and d is the thickness of the material. For bovine lenses a=1360/cm at 193 nm, a value close to that of human eye lenses. Thus 90% absorbance product of a and d equal to 1.0 i.e., d=1/1360=7.35 um (microns). For thicknesses 10 times this value, the transmission through the lens would be 1 part in 10 billion. Thus, it appears that the lens will totally protect the retina of the eye from 193 nm UV. However, very low levels of 193 nm UV, such as might be able to penetrate the cornea and aqueous humor could possibly damage the lens in a manner similar to longer wavelength UV radiation causing cataracts. Therefore, precaution to avoid exposing the lens to any amount of 193 nm UV is either necessary or desirable.
Some experimentation with in-vivo eyes using total corneal surface ablation with 193 nm UV has shown a tendency towards some hazing/cloudiness of vision. The clouding effect suggests that the angle of impingement of UV may be critical.
There presently exist a number of techniques to modify corneal shape using lasers for purposes of refractive correction. These include radial keratotomy, lamellar keratectomy correction (keratomileusis), and direct corneal shaping via ablation. Baron, is U.S. Pat. No. 4,461,294, implements the first of these (radial keratotomy) by imbedding laser light absorbing particles in a radial pattern, then vaporizing these particles by laser radiation to produce the radial scar tissue characteristic of the technique. Trokel, et. al., discussed above, and European Patent No. 0 151 869 discuss full surface corneal ablation using the excimer laser. In the Trokel paper and the European patent, no means for protecting of critical eye tissue is given. Belgorod, in U.S. Pat. No. 4,724,522, describes an apparatus and method allowing full protection of eye tissue.
The present invention is uniquely distinguished from the present known prior art in regard to both apparatus and method: In Baron, the method is limited to performing radial keratotomy which mandates the formation of scar tissue (eschewed in the present invention); also, the opaque contact lens therein plays no role in corneal shaping, serving only to shield the eye from laser radiation; and the reference to fiber optics is solely for delivering radiation along the radial slits. In Belgorod's apparatus, the system of mechanical rotational and linearly transrotational mirror elements is inherently complex--requiring high precision in construction and application to achieve the desired ablative sculpting by the excimer laser. This contrasts with the present invention, which rather than sculpting the cornea one area at a time, operates simultaneously over the total cornea, doing so with minimal precision requirements.